The DMCA isn't good, but there are lots of farm equipment manufacturers, auto companies, etc that don't sue their customers. The difference between Saab and John Deere in this case is that John Deere is going out of its way to behave in an unethical manner and screw its customers over.
What I'm really arguing against is this subtle subtext that says John Deere's actions are the fault of the government, and that if only the government wasn't around this kind of crap wouldn't be happening. A long history of anti-competitive, anti-consumer behavior by corporations that think they can get away with it says otherwise. Repeal the DMCA, but also create legislation protecting the consumer's right to repair, modify, sell, and otherwise control equipment that they legally purchased.